11 — Motion and Interaction System

Apply meaning-first CSS animations (scroll-driven, View Transitions, @starting-style) with mandatory prefers-reduced-motion on every animated surface.

Motion serves one of three purposes

1. Feedback — confirm user action (button press, form submit, save)
2. Continuity — preserve spatial context across state changes (page transition, modal open)
3. Delight — express brand personality (hero parallax, signature reveal)

Anything else = AI slop; cut it.

3-Tier Hierarchy

• Tier 1 — Functional — feedback on every interaction (hover, focus, active, tap). Duration 100–200ms. Transform/opacity only.
• Tier 2 — Choreographic — page transitions, modal entrance, section reveal. Duration 300–500ms. View Transitions or @starting-style.
• Tier 3 — Cinematic — hero parallax, signature reveal, scroll-driven storytelling. Duration ≥600ms. Scroll-timeline.

Never stack 3 tiers on same surface. One cinematic per page.

Mandatory prefers-reduced-motion

EVERY animation MUST honor prefers-reduced-motion: reduce — snap to final state, never hide content.

@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
    scroll-behavior: auto !important;
  }
}

View Transitions API

• Same-document SPA: document.startViewTransition(() => updateDOM())
• Cross-document MPA: @view-transition { navigation: auto; } in CSS
• Per-element: view-transition-name: hero-image; on persistent elements
• Chrome 126+, Safari 18+, Firefox 144 (Oct 2025)

::view-transition-old(root), ::view-transition-new(root) {
  animation-duration: 0.3s;
  animation-timing-function: cubic-bezier(0.4, 0, 0.2, 1);
}

Scroll-Driven Animations

• animation-timeline: scroll() (root scroller) or view() (element-in-viewport)
• Off-main-thread on Chrome stable + Safari 26 (2025)
• Firefox unsupported — pair with prefers-reduced-motion AND duration:1ms fallback

@keyframes fade-up { from { opacity: 0; transform: translateY(20px); } to { opacity: 1; transform: translateY(0); } }
.reveal { animation: fade-up linear; animation-timeline: view(); animation-range: entry 0% cover 30%; }

@starting-style (DOM-insert animation)

Baseline 2026 — animates from explicit starting state to default state when element enters DOM.

.toast { opacity: 1; transform: translateY(0); transition: opacity 0.3s, transform 0.3s; }
@starting-style { .toast { opacity: 0; transform: translateY(-10px); } }

Container Scroll-State Queries (Baseline 2026)

@container scroll-state(stuck: top) — replaces JS scroll-listener-based sticky styling.

.nav { container-type: scroll-state; }
@container scroll-state(stuck: top) { .nav { box-shadow: 0 4px 12px rgba(0,0,0,0.4); } }

Micro-Interactions (Tier 1 patterns)

• Button press — transform: scale(0.98) on :active, 100ms
• Hover — transform: translateY(-1px) + color shift, 200ms
• Focus-visible — 3px brand-accent ring, 2px offset, 0ms (instant)
• Tap — haptic feedback via navigator.vibrate(10) if supported
• Toggle — animated check/cross morph via <svg> path interpolation
• Loading — pulse 1.2s ease-in-out infinite

Stagger sequences (no JS needed)

.list-item { animation: fade-up 0.4s ease-out backwards; }
/* OR use sibling-index() (Baseline 2026) */
.list-item { transition-delay: calc((sibling-index() - 1) * 80ms); }

Performance constraints

• Animate transform + opacity ONLY on hot paths
• will-change sparingly (transform, opacity only when actually animated)
• Drop GPU layers after animation completes
• INP target ≤100ms per _kernel/standards.md#cwv — animations must not block input

Interaction polish (every interactive element)

Per 10-experience-and-design-system § 4-state distinction:

• Default → neutral
• Hover → underline-sweep + color shift + translateY(-1px)
• Focus-visible → 3px brand-accent ring 2px offset (distinct from hover)
• Active → scale(0.98) + immediate color confirm

Audit gate: Playwright cycles each interactive through 4 states → diff ≥3px pixel-difference or fail.

Banned motion

• ❌ Uniform fade-in on every element (AI slop tell)
• ❌ Parallax on every section (one cinematic per page)
• ❌ Spinning loaders that don't progress
• ❌ Auto-playing video with sound
• ❌ Carousel auto-rotate (manual swipe only — accessibility)
• ❌ Scroll-jacking that breaks browser back button
• ❌ Animations that block input (INP >100ms)
• ❌ Easter eggs on critical conversion paths

See submodules: view-transitions.md, scroll-driven.md, micro-interactions.md, reduced-motion.md, build-breaking-rules.md.